1. Field of the Invention
The present invention relates to a current leakage detector for detecting a current leakage of a motor drive circuit in a construction machine, e.g., a hybrid shovel or a battery shovel, in which a motor is driven by a battery.
2. Description of the Related Art
Hitherto, regarding the technology for detecting a current leakage in a battery-loaded vehicle such as a hybrid car, there is known a technique of applying a test voltage, e.g., a sine wave or a pulse wave, between a circuit bus and a machine body (ground) and measuring a crest value of the test voltage, as disclosed in Patent Document WO2007/007749.
The known technique utilizes the fact that, if a current leakage occurs due to, e.g., an insulation failure, the crest value of the test voltage is reduced to be lower than a reference value due to generation of current leakage resistance. The occurrence of the current leakage is determined when the crest value lower than the reference value is detected.
However, when the known technique is applied to a construction machine (e.g., a shovel which is taken as an example in a description below), the following problem arises.
In the shovel, an upper swivel structure provided with a working attachment is mounted on a crawler-type lower traveling structure to be able to swivel (rotate) about an axis normal to the ground surface.
A swivel motor for swiveling the upper swivel structure is driven through an inverter in a hybrid shovel that utilizes engine power and battery power in a combined manner, or in a battery shovel that utilizes only a battery as a power source.
The inverter includes a plurality of switching devices. In a swivel operation, the switching devices are turned on to bring the inverter into an operated state, whereby electric power is supplied to the swivel motor.
Speaking contrariwise, unless the swivel operation is performed, the inverter is not operated and electric power is not supplied to the swivel motor.
Further, unless electric power is supplied to the swivel motor, the detection of the current leakage cannot be performed by using the above-described technique.
This gives rise to the problem that the detection of the current leakage cannot be performed in a swivel stopped state.
More specifically, the above-described problem is attributable to such setting in view of a situation specific to the swivel motor that, in the swivel stopped state, the upper swivel structure is positively held stopped on, e.g., a sloped road by a swivel brake to be avoided from swiveling following a downward slope due to its own load, while the supply of electric power to the motor is cut off for energy saving.
To cope with that problem, it is conceivable to perform the detection of the current leakage during a period in which the swivel motor is operated, i.e., during the operation. However, start/stop of operation of the swivel motor is frequently repeated, and a battery voltage is abruptly varied correspondingly. Therefore, such a solution increases a possibility of erroneous detection, for example, that a current leakage state is determined in spite of a current leakage being not actually generated.
The above-described problem is experienced in not only the swivel motor, but also in all of other motors used in shovels or other construction machines in which the motors are driven through inverters and the supply of electric power is cut off in a motor stopped state.